1 Corinthians 9:16

"One point, which I believe is essential to a minister's faith, is that we believe in our own commission to preach the gospel... He who doubts as to whether he is sent of God, goes hesitatingly; but he who is certain of his call from above demands and commands an audience; he does not apologize for his existence, or for his utterances; but he quits himself like a man, and boldly speaks God's truth in the name of the LORD. He has a message to deliver which he must deliver, for woe is unto him unless he preaches the gospel." - Spurgeon.